The understanding of the role of natural polysaccharides in controlling microbial populations by selective enhancement or inhibition is emerging. The field of biopolymers has evolved significantly due to the versatility of their applications and the greater understanding of their function in many natural symbiotic processes. This development is particularly true for chitosan, a polysaccharide which is naturally abundant and has been successfully utilized in diverse areas of agriculture, wastewater treatment, food technology, animal food stock, paper/textile manufacture, biotechnology and biomedical devices and products.

Chitin, a polymer of N-acetyglucosamine, is a cellulose-like biopolymer that is the main component of crustaceans (e.g. shrimp, crab, lobster) and is also present in the exoskeleton and the cell wall of fungi, insects and yeast. Chitosan, a principle derivative of chitin, is formed from chitin by deacetylation in the presence of alkali. It is the mostly deacetylated form of the naturally occurring polysaccharide chitin. The process of removing the acetyl group from poly-β(1→4)-N-acetyl-D-glucosamine to poly-β(1→4)-D-glucosamine causes the formation of primary amines. Chitosan is not a single polymeric molecule, but a class of molecules having different molecular weights and different degrees of deacetylation. The structure of chitosan and chitin is shown in (1), where p and q are the fractional relationship of the monomers glucosamine and N-acetyl glucosamine, respectively, and are between 0 and 1. The sum p+q=1. For chitin, q->1 and the molecule is nearly fully acetylated. Molecules where q<0.5 are considered chitosan. A key feature of chitosan is the positive charge of its amino group (—NH3+) when pH is below its pKa (˜6.3). When the pH=pKa, 50% of the amines are positive. The fraction of amines that are positive increases exponentially as the pH decreases and decreases exponentially as the pH increases, by the Henderson-Hasselbach equation
  pH  =            pK      a        +          log      ⁢                                    [                          NH              2                        ]                                [                          NH              3              +                        ]                          .            At lower pH, native chitosan forms a polycationic structure that can interact with anionic compounds and macromolecular structures. While very low molecular weight chitosans with high degree of deacetylation are soluble at physiological conditions, most chitosans lack positive charge and are only soluble in acidic conditions. For example, chitosan is highly soluble in aqueous acetic acid.
Chitosan has numerous biological properties, including antimicrobial activity, hemostatic activity, acceleration of wound healing, tissue-engineering scaffolds, drug delivery, and antitumor activity. Additionally, chitosan, when biological burden is removed, is biodegradable and biocompatible with low toxicity to mammalian cells. It is also important to note that bacteria generally should not develop chitosan resistance.
Chitosan's unique biological properties make it medically important. Consequently, chitosan has been developed for a variety of applications using these properties such as biodegradability, non-toxicity and antibacterial activity against a broad spectrum of microorganisms. However, the use of chitosan is limited because of its insolubility at neutral and physiological pH. The present invention overcomes the limitations of the prior applications and methodologies of making and using chitosan, as discussed below.
It is well known that biopolymers that are natural polycations have a tendency to be antimicrobial. Defensins, for example, are small cationic polypeptides with antibacterial properties that are produced naturally by the human body. See Viljanen et al.; Effect of small cationic leukocyte peptides (defensins) on the permeability barrier of the outer membrane; Infect. Immun. September; 56(9): 2324-2329 (1988). Efforts to reproduce the effectiveness of natural polycations as antibacterials with limited toxicity have been met by limited success. The complexity of understanding the antibacterial properties results from the multiplicity of interactions required between the polycation and the bacteria. Some of the important factors that influence the antibacterial properties include degree of charge, distribution of charge, molecular weight, degree of hydrophobicity and placement of the charge relative to the polymer backbone. The magnitude of this latter effect is easily conceptualized from the experiments comparing poly-ε-lysine vs poly-α-lysine. See Shima et al.; Antimicrobial action of epsilon-poly-L-lysine. J. Antibiot; 37:1449-1455 (1984). Poly-ε-lysine has the positive charges located on the α-amine in close proximity to the linear polymer backbone. Poly-α-lysine has the positive charges located five bond lengths from the linear polymer backbone. Poly-ε-lysine was more active as an antibacterial than poly-α-lysine against 20 of 22 tested bacterial species. They also determined an optimal molecular weight. Additionally, they showed the importance of the positive charge by removing the effective amine by reaction with a variety of carboxylic acids with no positive charge. Furthermore, they showed that small oligomers of poly-ε-lysine interfered with protein synthesis in E. coli more than poly-α-lysine. These results suggest a complicated interplay between charge, molecular weight, and charge placement on antibacterial effectiveness.
Nonfunctionalized Chitosan and Salts
While several mechanisms have been proposed for chitosan's antimicrobial activity, the exact mechanism is still unclear. The currently accepted antimicrobial mechanism is based on the interaction of the positively charged chitosan with the negatively charged residues on bacterial cell surface. It is believed that this charge interaction alters bacterial surface morphology and either damages the membrane to induce membrane permeability that causes leakage of intracellular substances (e.g. electrolytes, proteins, nucleic acids, glucose, and lactate dehydrogenase), or develops an impermeable layer around the cell and prevents nutrients from entering the bacteria. Another proposed mechanism suggests that positively charged chitosan interacts with cellular DNA through chitosan penetration into the cells, consequently acting as a barrier to RNA and protein synthesis.
Chitosan or chitosan derivatives that are described with antibacterial activity are considered to be bacteriocidal or bacteriostatic, often with little distinction between the two mechanisms. A bacteriocidal material kills the live bacteria or some fraction therein. A bacteriostatic material hinders the growth of bacteria or some fraction therein but does not imply whether or not bacteria are killed or their growth inhibited.
Chitosan's antimicrobial activity is affected by several factors such as source of chitosan, molecular weight (MW), degree of deacetylation (DD), chitosan concentration, pH, temperature, solution cations and polyanions, bacterial species and the phase of bacterial growth. A variety of studies have demonstrated the need for positively charged or other soluble chitosans in attempts to address the mechanism of activity. See Muzzarelli et al.; N-(carboxymethylidene) chitosan and N-(carboxymethyl) chitosan: novel chelating polyampholytes obtained from chitosan glyoxylate. Carbohyd. Res. 107, 199-214, (1982); and Moller et al.; Antimicrobial and physicochemical properties of Chitosan-HPMC-based films; J. Agric. Food. Chem. 52, 6585-6591 (2004).
Studies on the mechanism of bactericidal activity of chitosan acetate suggest that cell membranes of Escherichia coli and Staphylococcus aureus are damaged by the chitosan and become permeable, likely a result of the electrostatic interactions between the positive amines on the chitosan and the negative phosphoryl groups of the phospholipids comprising the cell membrane. See Liu, H. et al., Chitosan kills bacteria through cell membrane damage. Internat. J of Food Microbio 95, 147-155 (2004). Another reference discloses the use of O-carboxymethylated and N,O-carboxymethylated chitosans as antibacterials due to the compounds' ability to bind to DNA and the subsequent inhibition of DNA transcription. See Liu et al.; Antibacterial action of chitosan and carboxymethylated chitosan; J. Appl. Poly. Sci. 79, 1324-1335 (2001). Another reference discloses a comprehensive survey of the proposed mechanisms of antimicrobial activity including antifungal, antibacterial and antiviral properties (highlighting the influence of MW, pH, polynucleotide binding, cell permeability, binding of essential minerals by chitosan, and its limitations above pH 6.5). See Ravbea et al.; Chitosan as antimicrobial agent application and mode of action; Biomacromolecules 4(6), 1457-1465 (2003). Yet another reference discloses that water soluble chitins and chitosans without sufficient positive charge or MW are not antibacterial, whereas insoluble, high MW chitosans are effective if placed in acidic media, again suggesting that positive charge plays a role in mediating bacteriocide. See Qin et al.; Water-solubility of chitosan and its antimicrobial activity. Carbohydrate Polymers; 63: 367-374(2006).
A careful study using temperature and a variety of pH's showed the dramatic effect of the degree of protonation, ie positive charge, on the antibacterial activity of chitosan against E. coli. For pH's between 5 and 9, they showed a dramatic and continual increase in cell death as the pH increased, with nearly no activity at pH 9. See Tsai et al.; Antibacterial Activity of Shrimp Chitosan against Escherichia coli; J. of Food Protection 62(3); pp. 239-243 (1999).
Several patents disclose applications of nonfunctionalized chitosan for absorbent materials, drug delivery, biocompatible/bioabsorbable materials, hemostatic agents, filters, textiles, crosslinked gels, as a carrier for soluble or active antibacterial and antimicrobial agents, as a chelating and flocculation method for metals in water, as biodegradable and/or edible films, shellacs and sheets, as well as an odor control agent, hydrophilic absorbent and biodegradable/biocompatible structural support for co-derived scaffolds. Chitosan is broadly represented in the patent and academic literature in the insoluble form.
Chitosan of significant MW is insoluble at physiological pH. However these references differ from the present invention by disclosing soluble chitosan compounds formed by intentional breakdown of molecular weight. U.S. Pat. No. 5,730,876 to You et. al discloses a method for fractionating low molecular weight, soluble chitosans through reaction with enzymes and acid and subsequent ultrasonic membrane filtration to bring the MW from over 100 kDa to 10,000 kDa or less. Reducing molecular weight is a common way to improve solubility of chitosan, but it does nothing to increase the positive charge on the polysaccharide. Patents disclosing the production of water soluble chitosans teach the desirability of formulations that are soluble at pH 7. In order to make such chitosans that are not derivatized, the molecular weight must be decreased dramatically, the degree of deacteylation high (depending on the MW) and may or may not have antibacterial effects, depending on the composition of the additives. For example, U.S. Pat. No. 4,532,134 to Malette et al. discloses a water-soluble chitosan that is used to treat wounds as a hemostatic agent. These low MW, low positive charge nonfunctionalized chitosans are not related to the current application.
Chitosan Mixtures (Compositions)
Many of the prior art references disclose the uses of mixtures of chitosan and atoms, chemicals (natural or synthetic), polymers or polypeptides which impart new properties to the entire composition including antibacterial or antimicrobial properties. Mixtures are a homogenous (same phase) or heterogeneous (different phases) composition of chitosan and any small molecule, polymeric molecule, or any form of solvent, stabilizing agent, or polyion. Chemically, physically, and functionally, heterogenous and homogeneous mixtures are not identical to single molecule comprised of components of the mixture.
For example, a number of patents discuss mixtures comprised of chitosan and these other agents, but do not constitute a compound or molecule. For example, US patent application publication US/2004/0104020 A1 to Davidson et al. discloses chitosan compositions for hair care, odor control, blood management, fabric treatment, plant care, water purification and drug delivery using a network of nano-sized fibers. This reference is not directed to antibacterials but is directed to the broad uses of chitosan and the mixtures of chitosan. This serves as one example of any number of nonfunctionalized chitosan compositions reflecting its broad usage independent of or including antimicrobial properties.
The prior art also discloses polycations that are useful in a number of broad formulations with or without chitosan. Chitosan and arginine or lysine, positively charged amino acids, or polymeric combinations of poly-arginine or poly-lysine, usually in the L stereo isomer, of the same often appear simultaneously on these lists as cationic biopolymers, but are used independently and not dependent on the combined properties for functionality. Examples of these include the following. U.S. Pat. No. 5,614,204 to Cochrum discloses “selective vascular embolization” utilizing an occlusion agent that may be alginate, chitosan or poly-L-amino acid. Poly L-amino acids include poly-L-lysine, poly-L-arginine, poly-L-glutamic acid, poly-L-histidine, poly-α-D-glutamic acid or a mixture of the above. As a mixture and as an occluding agent, the components of this composition teach away from the applications and activities and compounds of our application. See also U.S. Pat. No. 5,510,102 to Cochrum. U.S. Pat. No. 4,749,620 to Rha et al discloses microencapsulation including chitosan as a polymer selected from a group consisting of chitosan and poly-L-lysine. See also U.S. Pat. No. 4,744,933 to Rha et al. The utility and composition of the above references teach away from the present invention as discussed below. Japanese Patent 10175857 to Sekisui Chem. Co. Ltd. discloses a healing agent comprising a commercial ointment with a carbohydrate that is a mixture of positive material in a hydrocarbon base. This reference is directed to the addition of arginine, glutamate and their derivatives to the ointment, followed by the same commercial ointment with the addition of chitin, chitosan and their derivatives to ointment. Mutual benefit is not noted. US 2004/0103821 A1 to Shobu, et al., discloses a food and/or medicament coating of an insoluble shellac comprising a basic amino acid (e.g. arginine) and/or chitosan. This reference is however directed to an insoluble mixture.
Interestingly, chitosan and the positively charged amino acids appear in the patent literature not in combination, but due to their similar constituency as positively charged polycations in groups where they co-reside. See, for example, US Patent Publication US 2004/0028672 to Bjorck, et al. which discloses methods for identifying agents including L-arginine and chitosan, for treating chronic and acute microbial infections. This patent discloses polycations that may be screened for antimicrobial activity. Additionally, this reference is directed to only homopolymers of arginine or chitosan individually.
Chitosan and Antivirals or Antibacterials
Because chitosan is often used as a drug carrier, much of the patent literature teaches to the adhesive and biocompatible properties of chitosan and of chitosan's role as a component in compositions that contain antiviral or antibacterial active agents. For example, U.S. Pat. No. 6,465,626 October 2002 to Watts et al teaches that chitosan is one of many adhesive materials that can be used in pharmaceutical applications. The antiviral component is an agent ICAM-1.
Similarly, other patents teach to mixtures of low molecular weight chitosans that are soluble, but in compositions that antimicrobial activity only in the presence of another molecule, an active antimicrobial agent. For example, U.S. Pat. No. 5,730,876 You et al. teaches separation and purification of low molecular weight chitosan using multi-step membrane separation process and only claims antibacterial properties in the presence of elecampane (Inula Helenium L.) root extract. The active antimicrobial agent is the elecampane. Further, the teachings of U.S. Pat. No. 6,521,268 to You et al., disclose a “natural cell carrier” of water-soluble chitosan and elecampane extract—antibacterial, anti-inflammatory and broad antibacterial spectrum for food, cosmetics and medicine, and discusses the antibacterial mechanism as chitosan binding to telechoic acid in all bacterial membranes. It is the carrier, but also adjuvant to the true antibiotic and inflammatory elecampane. This patent teaches the role of chitosan in binding a cell membrane which supports the need for a water-soluble chitosan, but limits the membrane association to a telechoic acid. The patent teaches away from a high molecular weight antibacterial chitosan derivative.
Chitosan Salts
Chitosan can be prepared in an acidic solution and precipitated with active salts. For example, U.S. Pat. No. 4,957,908 to Nelson teaches that chitosan salts can be useful to help impart desirable properties, in this case for antimicrobial activity that does not adsorb quickly into the skin and can be used for dermatological items such as soaps and shampoos. Zinc and sodium pyrithione are used as antifungals and antibacterials, but absorb into the skin. By making a particular salt of the chitosan, they can make chitosan pyrithione that is as effective as their original material but dissolves more slowly. Similarly, U.S. Pat. No. 5,015,632 to Nelson discloses a salt of chitosan that is to provide slow release of an anionic salt, pyrithione, from films. It is used as an antimicrobial agent in dermatological items, but derives its properties from the pyrithione. This chitosan salt has the same antibacterial efficacy as the sodium salt of the same anion and thus teaches away from the present invention. U.S. Pat. No. 5,300,494 to Brode II et al. disclose a series of chitosan salts (in particular lactate salts) that act as carrier films for pharmaceutically active drugs (particularly quaternary ammonia compounds and salts). As thin films that retain moisture and the ability to retain and slowly deliver a drug, in this case an antiviral. These serve as examples of any chitosan salts that bear antimicrobial capacity due to the anionic salt, a salt mixture with an antibacterial salt rather than as the free compound.
U.S. Pat. No. 6,844,430 B2 to Pesce et al. disclose the use of aminopolysaccharide salts for the control of odor in sanitary products, diapers et al. and where a preferred saccharide is chitosan. The chitosan is prepared with a number of salts including the amino acids such as arginine and lysine. As salts, they are counterions and not part of the molecule but produce the desired biocompatibility. This example teaches to the ability to use salts to provide additional activity in a chitosan formulation, but does not teach the present invention.
Chitosan Derivatives
Interest has developed over the years in controlling the properties of chitosan by performing chemical modifications on the polymer backbone. For chemical reactions that retain the polymeric form of the chitosan, there are only two types of fairly highly reactive moieties on the monomers: the hydroxyl group and the amine group. The two hydroxyl groups have slightly different reactivity but can be functionalized by hydroxy active agents at high pH on either the acetylated or deacetylated monomers of the chitosan. The primary amine of the deacetylated monomer of the chitosan is available for reaction at moderate pH above 6 or so where a significant number of the amines are deprotonated. These chemistries provide new chitosan compounds bearing different properties from the original chitosan polymer.
Hydroxide Chemistry
Other patents teach the desirability of functionalization of chitosans to achieve particular goals. For example, US Patent Publication US 2003/0181416 A1 to Comper, discloses primarily Dextran which when sulfonated is effective in vivo in the treatment or prevention of viral, bacterial and parasitic infections. This reference teaches that many other polysaccharides are not antimicrobial or antiviral and teaches the desirability of controlling the molecular weight of polysaccharides for optimal in vivo and in vitro microbial activity. This reference also teaches the desirability of controlling the pH to improve deliverability of the active ingredients. The sulfated polysaccharides include two chitosan derivatives where the antimicrobial properties are imparted by negatively charged sulfates.
Carboxyalkylated chitosan derivatives, sulfonyl chitosan derivative, carbohydrate-branched chitosan derivatives, chitosan-iodine complexes and other miscellaneous derivatives were also developed. See Muzzarelli et al.; N-(carboxymethylidene) chitosan and N-(carboxymethyl) chitosan: novel chelating polyampholytes obtained from chitosan glyoxylate. Carbohyd. Res. 107, 199-214 (1982); Chen et al.; Antimicrobial effect and physical properties of sulfonated chitosan; Advances in Chitin Science, Vol III, 278-282 (1998); Yalpani et al.; Antimicrobial activity of some chitosan derivatives, Advances in chitin and chitosan; 543-548 (1992); U.S. Pat. No. 5,538,955 to De Rosa et al (1996); and Muzzarelli et al.; Fungistatic activity of modified chitosan against Saprolegnia parasitica; Biomacromolecules, 2, 165-169 (2001). The limited application and range of these derivatives teach to the need for more thoughtful application of chemical knowledge to the control of solubility and antibacterial properties.
Amine Chemistry-Quaternization
The primary amine on the glucosamine monomer of chitosan can be the basis for a number of reactions, the most important in the literature being quaternization. To quaternize the amine on chitosan, three additional groups must be added, taking the primary amine to a quaternary amine with a permanent positive charge. A quaternary amine is fairly electrophilic, but can remain stable in the absence of any available nucleophiles. Many of these derivatives add functional groups or modify the carbohydrate with non-biological moieties that render the molecule different than any naturally occurring molecule, and thus the toxicity of the molecule is unknown.
Much of chitosan chemistry has centered on the reactive amine that results from the deacetylation process of chitin. A quaternized chitosan derivative was developed by introducing quaternary ammonium salts onto the chitosan backbone. See Kim et al.; Synthesis of chitosan derivatives with quaternary ammonium salt and their antibacterial activity, Polymer Bulletin. 38, 387-393 (1997) and Zia et al.; Synthesis and antibacterial activity of quaternary ammonium salt of chitosan; Carbohydrate Res. 333, 1-6 (2001). The antibacterial activity and water solubility of this derivative was increased with the decrease in the chain length of alkyl sustituent. Although there are other methods for producing and analyzing quaternized amines on chitosan, a sampling of methods is presented. See Hamman et al.; Effect of the type of base and number of reaction steps on the degree of quaternization and molecular weight of N-trimethyl chitosan chloride; Drug Dev. And Ind. Pharm 27(5), 373-380 (2001); Avadi et al.; Dimethylmethyl chitosan as an antimicrobial agent: synthesis, characterization and antibacterial effects; Eur. Polymer J. 40, 1355-1361 (2004); Sashiwa et al.; Chemical modification of chitin and chitosan 2: preparation and water soluble property of N-acylated or N-alkylated partially deacetylated chitins Carbohydrate Polymers 39, 127-138 (1999); and Sieval et al.; Preparation and NMR characterization of highly substituted N-trimethyl chitosan chloride; Carbohydrate Polymers 16, 157-165 (1998). The quaternary amine has its positive charge surrounded by bulky methyl or even more sterically constraining longer hydrocarbons, but clearly teaches to the importance of positive charge in the antibacterial properties and solubility of chitosan.
The prior art is rich with the synthesis and applications of chitosans with methodology to produce the quaternary amine. A series of patents teach the importance of solubility for materials having necessary activity at physiological pH. For example, a series of patents describe the creation of and use of the quaternary amines as foaming and stabilizing agents in cosmetic compositions. For example, U.S. Pat. No. 4,772,689 to Lang et al. discloses the quaternary chitosan derivatives with hydroxy and propyl substitutions on the amine that are used in cosmetic compositions for the treatment of hair or skin, characterized by a content of new quaternary chitosan. Also disclosed are the new quaternary chitosan derivatives per se as well as processes for their preparation. The chitosan derivatives have a good substantivity, particularly to hair keratin, and prove to have hair strengthening and hair conditioning characteristics. U.S. Pat. No. 4,976,952 to Lang et al. discloses a cosmetic agent for the treatment of the hair and skin that contains macromolecular surface-active, quaternary N-substituted chitosan derivatives with a variety of degrees of substitution and pendant groups on the amine. This invention also comprises chitosan derivatives distinguished particularly by their surface-active properties, for example, their foam-forming and emulsifying properties, and by their hair-setting and hair-conditioning effect. Similar to these are U.S. Pat. Nos. 4,921,949; 4,822,598 and 4,772,689 all to Lang et al. These describe the action of cation-active polymers, particularly polymers which have quaternary ammonium groups, as conditioning compositions in cosmetic compositions, particularly for the treatment of hair. Based upon a reciprocal action between their ammonium groups and the anionic groups of the hair, the cation-active polymers possess a great affinity for keratin fibers. These series of patents teach to the desirable interactions of polycations and polyanions for particular applications and the ability to modify chitosan to achieve and control those interactions.
Amine Chemistry-Acid Coupling
Amines are typically coupled to carboxylic acids using peptide coupling chemistry. These chemistries continue to be performed in organic solvents, primarily for the synthesis of polypeptides and short proteins. Only recently were many of the coupling agents modified to be active in aqueous solutions See Bioconjugate Techniques Greg. T. Hermanson (Elsevier, Academic Press: USA) (1996).
Few descriptions exist of chitosan derivatives N-conjugated with different amino acids. Jeon et al., describe low molecular weight chitosan polymers (less than 10,000 Da) with asparagine, glycine, alanine, aspartic acid, cysteine and methionine. However they do not disclose or describe the inherent solubility issues of their chosen amino acids. Rather Jeon et al. rely on inherent solubility provided by selecting low molecular weight chitosan, thereby limiting the applicability of their compounds. Additionally, Jeon et al do not utilize the positively charged amino acids nor do they disclose or describe any correlation between solubility and charge. It is important to note that Jeon et al. focus upon aspargine, an amino acid that is neutral at all pH's. Thus Jeon et al suggest that neutral amines at physiologic pH contribute to antimicrobial activity. This implication teaches away from the present invention. Furthermore, the coupling method disclosed by Jeon et al requires the use of N,N′-dicyclohexylcarbodiimide (DCC) as a coupling agent which is not water soluble. The reaction is preformed in 4:1 methanol:water mixtures with triethyl amine (TEA) as a base to bring the pH to 6.8. After reaction, deprotection is performed in trifluoroacetic acid to remove the boc protecting groups thereby further reducing the MW and producing a large distribution that is not addressed for the various chitosan products disclosed by Jeon et al. Furthermore, the activity disclosed in Jeon et al is not unlike the variability exhibited by low molecular weight chitosan. Thus, Jeon et al fail to teach control of higher weight chitosans, as per the present invention. See Jeon et al.; Effect of antimicrobial activity by Chitosan oligosaccharide N-conjugated with Asparagine. Microbial. Biotechnol. 11(2): 281-286 (2001).
It is known that chitosan is an avid coagulant and that chitosan of different molecular weights are utilized to induce clotting and provide hemostasis. See U.S. Pat. No. 4,394,373 to Malette et al. It is also important to note that there are scant disclosures of the desirability of producing arginine bound to chitosan as anticoagulation biomaterials. However, no disclosure is made of the fact that the methodology as disclosed, produces polymers of their chosen amino acids. Additionally, no disclosure is made of any manner of limiting the formation or coupling of polyamino acids to chitosan as is necessary for the formation of chitosan-arginine. As is well understood by one of ordinary skill, that a mechanism for limiting the formation or coupling of poly amino acids to chitosan is essential to the formation of chitosan-argine. Additionally, no disclosure is made as to the relevance of charge density to antibacterial properties of chitosan-arginine. Thus, in the absence of such a teaching, severe doubt must be raised as to the probability that the formation of chitosan-arginine has indeed occurred and in such desirable quantities to perform the described utility. See Liu et al.; A chitosan-arginine conjugate as a novel anticoagulation biomaterial. J. of Materials Science: Materials in Medicine 15:1199-1203 (2004).
Chinese Patent 1519035A1 provides limited disclosure of the ability to make chitosan-arginine for the purpose of biomedical polymers for in vivo implants. This reference is directed to the inhibition of hemagglutination by chitosan-arginine based upon their disclosure that arginine derivatized chitosan has a longer hemagglutination time than chitosan alone. However, it is important to note that several patents and purchasable products directly contradict the assertion that chitosan based materials can inhibit hemagglutination. See U.S. Pat. No. 4,394,373 to Malette et al. and U.S. Pat. No. 6,162,241 to Coury et al. See also, Klokkevold et al.; Effect of chitosan on lingual hemostasis in rabbits; J. Oral and Maxillofacial Surg. 49(8): 858-863(1991).
The methodology of Chinese Patent 1519035A1 includes peptide coupling that is well known in the literature, and thereafter subjecting the arginine-chitosan product/pre-product to a magnetic field. No disclosure is made as to the relevance of the magnetic field. The references cited by the presently discussed publication evidence an absence of interest in chemically protecting the α-primary amine on the arginine and thus, an inability to control the chemistry at that amine. The importance of control of the primary active amine results in the ability to control coupling, as discussed above. Thus, an active primary amine that is similar in activity to the amine on the chitosan will inevitably react with the coupling agents to react with the activated carboxylate group of other arginines resulting in poly-arginine, either attached to the chitosan or copolymerized in solution. A method of controlling activity on this reaction site is necessary to the production of the chitosan-arginine products as disclosed, and an absence of such a disclosure must lead one of ordinary skill to the conclusion, that the present publication has only addressed the desirability of the formation of such compounds, and not the actual disclosure of such compounds. Additionally, this reference requires insolubility, a characteristic that teaches away from the present invention, particularly antibacterial applications.
A number of patents teach to the addition of amino acids for desirable properties. U.S. Pat. No. 4,908,404 to Benedict et al. discloses that a polymeric backbone of polypeptides having primary or secondary amines can be functionalized with combinations of synthetic amino acids for creating strong bioadhesive materials that are compatible with living tissues. While general teachings of the desirability of coupling multiple components on a cationic backbone are disclosed, the reference is markedly distinguished from the present invention as discussed below.
Further addition of short polypeptides has been attempted on chitosan to stimulate cell growth or adhesion. See Ho et al.; Preparation and characterization of RGD-immobilized chitosan scaffolds; Biomaterials 26: 3197-3206 (2005)]. Ho et al describe attaching the four amino-acid polypeptide RGDS through a carbodiimide coupling scheme for the purposes of coupling free amines of the R (arg) peptide as well as the amine on the chitosan. Ho et al also disclose coupling of short polypeptides to an insoluble chitosan matrix for chitosan scaffolds. Another reference discloses coupling a longer polypeptide using an activated reagent that removes the need for protecting the amine on the amino acids. See Masuko, et al.; Chitosan-RGDSGGC conjugate as a scaffold material for musculoskeletal tissue engineerin; Biomaterials 26: 5339-5347 (2005). This same coupling agent is also taught in U.S. Pat. No. 7,053,068 to Prinz. Prinz also discloses a method for reacting the amines on chitosan with iminothiolactones, which impart a positive charge to the chitosan and provide an excellent coupling group for thiol chemistry. These can be made to gel or crosslinked for controlled release drug delivery. This reference provides a substantial teaching of the strength of active amine groups and the need for careful control of the chitosan reactive amine for practical coupling.
Textiles
Additional studies include the different responses by bacteria subjected to a range of molecular weight chitosans prepared on textiles. Though very limited in scope, additional variables that are considered in a small number of bacterial studies are; degree of deacetylation, pH, cations and anions present in solution. See Shin et al.; Molecular weight effect on antimicrobial activity of chitosan treated cotton fabrics; J. Appl. Poly Sci., 80, 2495-2501 (2001); and Lim et al.; Review of chitosan and its derivatives as antimicrobial agents and their uses as textile chemicals, J. of Macromolecular Sci. C43(2), 223-269 (2003). Wound treatments have also been addressed by utilizing a combination of chitosan with silver sulfadiazine for wound dressings and burns. See Mi et al.; Asymettric chitosan membranes prepared by dry/wet phase separation: a new type of wound dressing for controlled antibacterial release; J. Membrane Sci.; pp. 212, 237-254 (2003). Additionally wound dressings have also utilized chitosan in combination with glycerol, chitin and ethylene oxide. See Marreco et al.; Effects of different sterilization methods on the morphology, mechanical properties and cytotoxicity of chitosan membranes used as wound dressings, Wiley periodicals, 268-277(2004).
Chitosan-Guanidine
The addition of a guanidinium group to a primary amine impart positive charge or polarity or to act as an intermediate step in chemical reactions has been developed primarily in organic solvents. Use of a known guanidinylation (guanilating) reagent, formamidine sulfinic acid, has been demonstrated in absolute methanol for small molecules containing primary amines. See Katritzky et al.; Recent developments in guanylating agents. ARKIVOC iv, 49-87 (2005); and Maryanoff et al.; A convenient synthesis of guanidines from thioureas; J. Org. Chem. 51, 1882-1884 (1986).
Very little has been reported in the literature for chemical functionalization of the primary amine on chitosan by direct guanidinylation. Due to chitosan's insolubility in organic solvents, syntheses are restricted to aqueous solutions. The effectiveness of chitosan functionalization by typical guanylating agents in water is a challenge, as these are somewhat different than the conditions under which these reagents were originally intended to operate. Reaction with formamidine sulfinic acid is an atom-economical reaction with no by-products, and the sulfinate provides the salt to the positive guanidinyl product.
The chemistry of another guanylating reagent, 1H-pyrazole-1-carboxamidine hydrochloride, has also been examined for primary amines. See Bernatowics et al.; 1H-pyrazole-1-carboxamidine hydrocholride: an attractive reagent for guanylation of amines and its application to peptide synthesis; J Org. Chem 52, 2497-2502 (1992). This reactant can be synthesized and characterized in gram quantities in a single synthetic operation.
The bioactivity of biguanides and chitosan with related biguanidinylations have been disclosed in the prior art. Japanese Patent 60233102 to Toshio discloses a chitosan derivative from dicyandiamine or dyanamide to produce a coagulating material and metal ion absorbant that has biguanide or and or guanidine groups. This reference is distinguished from the present invention because it fails to teach structure and does not provide disclosure of the relevance of solubility charge and antimicrobial properties of the compound.
Microbial Populations
A variety of debilitating diseases and syndromes are the result of poorly regulated microbial populations. Many symptoms of disease are produced by the concurrent bacterial infections that encroach upon weakened immune systems and tissues. A dramatic rise in use of common antibacterials has resulted in the concurrent rise of antibacterial resistant species. Wounds, lacerations and abrasions as well as burns and ulcers are dermal occurrences that are easily contaminated by a variety of environmental bacteria. Prosthetic joint sites where rubbing and abrasion often regularly occur are a common site for chronic infections. Furthermore, microbial imbalances in the gut are fairly frequent. One important example is the peptic ulcer, which is primarily caused by the acid-loving Helicobacter pylori, a gram negative bacterium. Inflammatory bowel syndromes, such as Crohn's disease or ulcerative colitis, are the result of the body's inability to control bacterial residence in the gut and the leakage of bacteria across the gut membranes. Peritonitis is an inflammation of the peritoneum and can result the dramatic translocation of bacteria across the gut walls and membranes.
Bacteria have been considered free floating organisms, but in the natural world, most bacteria (˜99.5%) aggregate in biofilms and behave differently than their planktonic forms. See O'Toole et al.; Biofilm formation as microbial development; Annual Review of Microbiology 54:49 (2000); Watnick et al.; Biofilm city of microbes; Journal of Bacteriology 182:2675 (2000); Stoodley et al.; Biofilms as Complex Differentiated Communities; Annual Review of Microbiology; 56:187 (2002). Bacterial biofilm formation is an industrial problem affecting water purification systems, heat exchangers and biological sensors. Biofilms serve as a continuous source of planktonic bacteria, which, when released from biofilms, seed formation of new biofilms in new locations.
Biofilms are also a major cause of human disease; chronic bladder infections, colitis, conjunctivitis, and periodontal disease are only a few among many well-established examples See Davies; Understanding biofilm resistance to antibacterial agents; Nature Reviews Drug Discovery 2:114 (2003). Biofilms problematically colonize medical devices such as catheters (e.g. urinary catheters are among the worst), contact lenses and artificial implants such as pacemakers, stents, dental and breast implants, and heart-valves among many others. See id. These biofilms are highly resistant both to clearance by the immune system and to antibiotic treatments.
Inside the body, biofilms serve as a protected source of continuously shed bacteria and biofilm fragments. The sloughed-off materials seed into the surrounding tissues and the circulatory system leading to recurrent acute infection. See Anderson et al.; Intracellular bacterial biofilm-like pods in urinary tract infections. Science 301:105(2003). In addition to releasing the extracellular matrix materials, the biofilm-resident bacteria also show protective behaviors (e.g. expression of multidrug efflux pumps) which may play an important role in evolution of multi-drug resistant nosocomial infection. For example, Pseudomonas aeruginosa, a common nosocomial pathogen and an adept biofilm-former, is multi-drug resistant at an alarming rate of 95% in the planktonic form.
Biofilm bacteria display profoundly decreased sensitivity to biocides and antibiotics, becoming 10-1000 fold more resistant than the same type of bacteria grown in planktonic culture. See Luppens et al.; Development of a standard test to assess the resistance of Staphylococcus aureus biofilm cells to disinfectants; Applied & Environmental Microbiology 68:4194 (2002). Controlling bacterial populations in biofilms is clearly a challenge.
The most common causes of diarrheal diseases are E. coli, Campylobacter jejuni and Shigella. There are no vaccines approved by the FDA to prevent infection. Intense psychological and physical stressors often lead to respiratory infections such as bacterial pneumonia and streptococcal infections as well as increased susceptibility to viral influenza. Escherichia coli are responsible for a variety of diarrheal and intestinal diseases. Diarrhea treated with ciprofloxacin substantially increases the antimicrobial resistance rates for multiple antibiotics, although they appear to return to pretreatment levels within a month. See Shannon et al.; Antimicrobial Agents and Chemotherapy; pp. 2571-2572 49:6 (2005). Outbreaks of E. coli poisoning due to improper food storage are not infrequent and can lead to high mortality rates due to the specific shiga-like toxins produced by strains such as the O157:H7. Other enteropathic bacteria with devastating effects include Campylobacter species, Salmonella species, Shigella species, and Vibrio species, which can produce gastroenteritis with severe diarrhea, nausea and vomiting.
A number of bacteria are associated with battlefield wounds but translate directly into the civilian population. Acinetobacter baumannii, a bacterium found in soil and water, has resulted in wound, respiratory, and bloodstream infections. The bacterium poses a danger due to its ability to survive on surfaces for up to 20 days and its apparent resistance to most known antibacterials. Acinetobacter is one of the most common gram-negative bacteria to colonize the skin of hospital personnel, potentially increasing the likelihood of nosocomial infection amongst other patients. A. baumannii can easily lose susceptibility to the antibiotics available. Only three drugs have been known to have exhibited efficacy against A. Baumannii. Imipenem carries a risk of seizure, amikacin, does not work for bone infections and has not been effective against some strains of the bacteria, and colistin, an antibiotic with severe toxic effects on the kidneys. See Aronson et al.; In Harm's Way: Infections in Deployed American Military Forces; Clinical Inf. Disease Volume 43: 1045-1051(2006).
Major complications of burn injuries include fluid loss and wound sepsis due to bacterial infections; a common cause is the Pseudomonas aeruginosa organism, which can be difficult to treat due to its resistance to antibiotics. Oral antibiotics, with the exceptions of the fluoroquinolones, are generally ineffective against most serious skin and soft tissue infections by P. aeruginosa. See Dale et al.; Therapeutic Efficacy of “Nubiotics” against Burn Wound Infection by Pseudomonas aeruginosa; Antimicrobial Agents and Chemotherapy; pp. 2918-2923, 48:8 (2004). Burrowing bacteria such as the Proteus are particularly difficult to treat in deep wounds and burns.
A variety of other pathogens have emerged as multi drug resistant. Klebsiella pneumoniae can cause nosocomial wound infections and is resistant to ampicillin. Many strains have acquired resistance to carbenicillin, quinolones, and increasingly to ceftazidime. The bacteria remain largely susceptible to aminoglycosides and cephalosporins. Cutaneous infection from Leishmania major generally results in chronic, painless skin lesions. Leishmania tropica and Leishmania infantum-donovani may be associated with visceralization and more chronic, reactivating illness. While treatment controls the clinical disease, it does not destroy the organism. Methicillin resistant Staphylococcus aureus (MRSA) is resistant to methicillin and other more common antibiotics such as oxacillin, penicillin and amoxicillin. MRSA infections occur most frequently among persons in hospitals and healthcare facilities who have weakened immune systems. From 1995 to 2004, the percentage of resistant bacteria in ICU patients has increased from <40 to 60% [National Nosocomial Infections Surveillance System, CDC].
In addition, viruses are an important set of microbes that infect almost any type of body tissue, including the brain. Often the use of antibiotics complicates viral infections. Most treatments for viral infections are preventative in the form of vaccines, as the vast majority of human viral infections are controlled by the immune system. However, materials are needed to prevent or treat serious viral infections or reduce viral infectivity.
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